Seismographs Record Blasts

January 30, 1996|By JOHN MAINES Staff Writer

For centuries, different cultures have had different theories on what caused the earth to shake below them.

Ancient Gabrielino Indians of Southern California blamed fighting between three turtles said to be holding up the Earth. Russian folklore talked of a god whose giant, flea-invested sled dogs scratched and caused houses to crumble. The Chinese blamed an underworld frog with a nervous twitch, while Peruvians considered the shaking to be the footsteps of a god.

And in modern-day South Florida? People who feel the rumble for the first time think ... earthquake!

"I stayed home from work one day. I felt it, and said, this is unbelievable. There was incredible shaking. I thought for sure it was a quake," said Randy Kittleson, of the Chapel Trails development in Pembroke Pines.

But he was wrong. What he felt were man-made blasts, caused by explosives used to prepare the ground for development.

Blasting, which triggered hundreds of damage complaints, has stopped temporarily. But the issue is coming up again in Pembroke Pines, Miramar, Davie and at the county level.

What should you do if blasting returns, and your house suddenly has cracks?

Build a homemade seismograph to detect tremors and record their impact.

"Research seismographs are very complicated. But they work on the same principle as the cheapest homemade devices," said John Watkins, a seismologist with the University of California at Berkeley. "My 6-year-old hated tremors," Watkins said. "But then I built him a small seismograph, and he couldn't wait until the Big One."

The earliest seismographs go back thousands of years, to tremor-prone China.

Douglas Weins, a professor of geography at Washington University in St. Louis, said Chinese emperors had a vase-like cylinders with dragon's heads coming out in all directions.

A marble was inserted in the mouth of each dragon, and even the smallest far-away quake would cause a marble or two to fall. It's said they could tell where the tremor came from by which marbles fell.

"Here, we worry about a crack in the wall of a house," Weins said. "There, I guess the emperor would see a marble fall and say, `Well, there goes another city.'''

The simplest detectors, while not scientifically precise, are based on models of early American seismographs, Weins said. But more complicated devices cost only about $60 to $70 and are fairly accurate, said Jill Johnston, spokeswoman for the Center for Earthquake Research and Information in Memphis.

"We measure earthquakes from thousands of miles away on a thing that costs 60 bucks," she said.

Two seismographs can easily be built at home for about $10 each.

One detects side-to-side motion, the other up-and-down shock waves. They work on the same principle as the most modern seismograph: A weight is suspended in a frame. When the earth shakes, the frame moves, but the weight essentially stands still. A measuring device attached to the weight records the movement on paper that's attached to the frame.

"Try them, measuring trucks on bridges, trains, plane vibrations, thunderstorms," Weins said. "Put them on a table and hit the table in different areas. You'll be surprised. You can tell the direction and size of the impact."

"Blasting measurements are different from tremors, but you can see them. We get them on our seismographs all the time," Weins said.